Set also SHLIB_OPENMP_CFLAGS on GHA

Author: csoneson

.github/workflows/R-CMD-check.yaml

@@ -108,6 +108,7 @@ jobs:
           export PKG_LIBS="-L$(brew --prefix gettext)/lib -lintl"
           echo "LDFLAGS += -L$(brew --prefix libomp)/lib -lomp" >> ~/.R/Makevars
           echo "CPPFLAGS += -I$(brew --prefix libomp)/include" >> ~/.R/Makevars
+          echo "SHLIB_OPENMP_CFLAGS=-fopenmp" >> ~/.R/Makevars
           R -e 'install.packages("data.table", type="source")'
 
       - name: Install R package dependencies